As shown in FIG. 13, a conventional crawler crane 1 comprises a traveling member located below a frame 11 and traveling using a crawler, and lateral pairs of (a total of four) outriggers A, B, C and D provided at a front and rear ends, respectively, of the frame 11 to ensure safety during operations (see JP2002-3172A).
Examples of a safety device against crane overturning include a moment limiter device using a microcomputer and a safety device against overturning comprising a flexing structure interposed between an outrigger main body and a ground panel, detecting means for detecting the amount of flexure, and control means for outputting an alarm or shutting off a hydraulic circuit when the amount of flexure exceeds a predetermined set value (see JP6-63577U).
A safety device against overturning implements predetermined overturning preventing means by using a load detector to detect the ground reaction to each outrigger, finding the ratio of the smallest of the sums of the ground reactions to every two longitudinally or laterally adjacent outriggers to the sum of the ground reactions to all the outriggers, and comparing the value of the ratio (safety) with a predetermined safety reference value (see JP10-72187A).
This safety device against overturning prevents overturning by executing the process described below.
The ground reactions Pa, Pb, Pc and Pd to the four outriggers A, B, C, and D are detected.
The sums of the ground reactions to every two longitudinally or laterally adjacent outriggers are calculated to find the minimum value Smin.S1=Pa+PbS2=Pb+PcS3=Pc+PdS4=Pd+Pa
(3) The sum of the ground reactions to all the outriggers is found.ΣPi=Pa+Pb+Pc+Pd
(4) Safety is determined.R=Smin/ΣPi
(5) The value for safety R is compared with a predetermined safety reference value R0. If R≧R0, the device determines that the crane is safe. If R<R0, the device determines that the crane may overturn to actuate an alarm lamp.
However, this safety device against overturning poses the problems described below.
In connection with the overturning performance of the crane, the crane has a fixed overturning moment. Accordingly, a rated load Wr regulating the upper limit of a lifting load W decreases with increasing working radius r.
The sum ΣPi of the ground reactions to all the outriggers is equal to the sum of the lifting load W and the weight of the machine body (fixed). Accordingly, an increase in working radius r and a decrease in rated load Wr reduce the sum ΣPi of the ground reactions to all the outriggers.
When the safety device against overturning outputs an alarm, the relationship between the safety R and the safety reference value R0 is R<R0. Since R=Smin/ΣPi, a decrease in the value of the sum ΣPi of the ground reactions reduces the minimum value Smin of sums of the ground reactions to every two adjacent outriggers at which value the alarm is output.
That is, an increase in working radius r reduces the minimum value Smin of sums of the ground reactions to every two adjacent outriggers at which value the alarm is output. This lowers the reference value for the reaction at which value an alarm for crane overturning is output. The reference value approaches zero.
When the minimum value Smin of sums of the ground reactions to every two adjacent outriggers at which value the alarm is output approaches zero, this means a short time interval between the output of the alarm and overturning. That is, a slight overload may cause the outriggers to float. Consequently, if the crane is violently operated with a large working radius r, inertia acting on a cargo or a boom may lower the value for safety R below the safety reference value R0. Then, the outriggers may float immediately after the alarm has been output. This may cause the crane to overturn.
Further, each of the outriggers A, B, C, and D of the crawler crane 1 comprises an attaching member 13 supported by the frame 11 using a rotatively moving shaft 12 so that the attaching member 13 is rotatively movable in a horizontal direction, a base end arm 15 supported by the attaching member 13 using a rising and lying shaft 14 so that the base end arm 15 can be freely raised and laid, an intermediate arm 17 supported by the base end arm 15 using a rising and lying shaft 16 so that the intermediate arm 17 can be freely raised and laid, a leading end arm 18 slidably fitted into the intermediate arm 17, a ground contact portion 19 pivotably connected to a leading end of the leading end arm 18, and an outrigger cylinder 20 provided between the attaching member 13 and the base end arm 15 to raise and lay the base end arm 15, as shown in FIG. 14.
With the safety device against overturning of the crawler crane 1, the load detector is commonly provided between the leading end arm 18 and the ground contact portion 19.
However, in this case, the electric wiring between the load detector and the calculating portion of the safety device against overturning must be laid through the sliding portion between the leading end arm 18 and the intermediate arm 17 and the rotative moving portions between the intermediate arm 17 and the base end arm 15, between the base end arm 15 and the attaching member 13, and between the attaching member 13 and the frame 11. Consequently, the electric wiring is cumbersome and is likely to be broken.
To avoid this problem, the load detector 2 may be provided at a base end of the outrigger cylinder 20 or a base end of the base end arm 15.
However, if the load detector 2 is installed at such a position, the force exerted on the load detector 2 is much stronger than the ground reaction acting on the ground contact portion 19.
For example, if the load detector 2 is provided at the base end of the outrigger cylinder 20, if the rising and lying shaft 14 at the base end of the base end arm 15 is defined as the center of a moment attributed to the ground reaction, the product of the ground reaction P acting on the ground contact portion 19 and the overhang distance La of the outrigger is equal to the product of the force F exerted on the load detector 2 and the distance Lb between the rising and lying shaft 14 and an attaching pin 21 of the outrigger cylinder 20. That is, the following equation can be given.P×La=F×Lb
Accordingly, the ratio of the force F exerted on the load detector 2 to the ground reaction P is:F/P=La/Lb.
Therefore, if the overhang distance La of the outrigger is 1.5 m and the distance Lb between the rising and lying shaft 14 and the attaching pin 21 of the outrigger cylinder 20 is 0.3 m, the force F exerted on the load detector 2 is five times as strong as the ground reaction P.
If the load detector 2 is composed of, for example, a load cell (see JP2001-220086A) having a strain gauge on a coil spring, the force F exerted on the load detector 2 increases, resulting in the need for a larger coil spring. This requires an increase in the size of the load detector 2.
However, the crawler crane 1 must be made compact to prevent an increase in the width of the crawler so as to meet the requirements for transportation using a transport vehicle. Thus, the size of the outriggers A, B, C, and D must be minimized. This limits the outside dimensions of the load detector 2, thus precluding the free selection of an installation position.
On the other hand, if a boom 5 is located on any of the outriggers A, B, C, and D, the crawler crane 1 is unlikely to overturn. That is, in this state, the crawler crane 1 does not overturn in spite of an excessive lifting load W. As a result, the boom 5 or the like may be overloaded and damaged.